RE: mosaic screen on c64

No 7708

But 7712 et 7709



-----Message d'origine-----
De : owner-cbm-hackers@musoftware.de
[mailto:owner-cbm-hackers@musoftware.de] De la part de Gerrit Heitsch
Envoyé : mardi 5 août 2014 18:54
À : cbm-hackers@musoftware.de
Objet : Re: mosaic screen on c64

On 08/04/2014 11:08 PM, didier derny wrote:
> Thanks, a friend bought a c64
>
> http://cbm8000.com/IMG_1387_2.png
> http://cbm8000.com/IMG_1385_2.png
>
> For now I only have a small image around the 6569...
>
> According to this small image it's either
>
> http://www.c64-wiki.com/index.php/File:C64_PCB_Assy250407RevA.jpg

Ok, so it's a 250407 board.

The image of the screen tells us a few things... First, the CPU starts up
and runs at least part of the ROM code, enough to program VIC so it attempts
to display something. The mosaic looks like the color RAM doesn't get
initialized (SRAMs like the 2114 start up with random
contents) and since there are no visible characters at all, it looks like
VIC doesn't see the char ROM.

So, my bet is the PLA or any MOS-TTLs. The latter were widely used on the
250407 and 250425 boards. If the board doesn't use 74LS257 but MOS 7708,
it's very likely that those are dead (They usually work for a short time
after power up and then crap out). Replace with 74LS257.

  Gerrit



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